At this point you should be around Lvl 11 with 39 Magic Penetration (+15% due Masteries) and around 200 Ability Power, now you should be able to deal (almost) true damage to the most enemy champions.

The Enemy's will now be forced to buy Magic Resist Items (if they dont want to die), to counter this you will focus on: Void Staff, this items allows you to continue deal a huge anmount of damage.
They might be will focus you due teamfights, for that you need to buy the Zhonya's Hourglass it allows you to deal full damage with your ultimate aswell as beeing untouchable for 2s, its a great item to counter any focus on you.

Note: Most of the games might be ending up after you bought Zhonya's Hourglass, if not keep on building the following item's: Abyssal Mask a great item to increase your damage aswell as increasing your Magic Resist and decreasing the Magic Resist of enemy champions which will increase your damage further more. Rylai's Crystal Scepter Huge HP/AP Ratio, and it adds a slow on your skills, which is awesome for chasing down a single enemy champion.

Items which may be good if facing huge anmount of CC on the enemy team: Quicksilver Sash awesome against any type of CC Banshee's Veil good against cc, also gives HP, but i dont like the face that we paying for MP on this item.

Team Work

Kennen has a huge impact on teamfights, he is able to stun/disable the entire enemy team.
But never initiate with Kennen you will be dead within 1s, always wait for your team to initiate.
If the enemy team initiate, stay back to not be focused, its also wise to stay behind a wall you can Flash over. they maybe dont expect you, Flash over and Lightning Rush through them, turn on Slicing Maelstrom if they focus you never forget to use Zhonya's Hourglass now watch 2s how your team is able to kill the entire enemy team :)

Summary

Kennen is in my eye's the strongest AP Champion, he's able to turn a whole teamfight around!